1. Field of the Invention
The present invention relates to resistance butt welding, and more particularly to a method of resistance flash butt welding of metals.
The invention is particularly useful for welding workpieces with relatively large welding surfaces, such as for welding workpieces with relatively large welding surfaces, and such as for welding structures from rolled ferrous metal, tubing, sheet metal at a relatively low set specific power of welding transformers.
2. Description of the Prior Art
Various resistance flash butt welding methods have been proposed wherein the speed of feeding the workpieces being welded toward each other is in the course of welding increased as a function of the current pulsation frequency (USSR Inventor's Certificate No. 182,265), or of the current pulse rate (Japan Pat. No. 46-8566/71), or of the voltage drop across the contact, i.e. across the welding zone or the spark gap (U.S. Pat. No. 3,790,739), or of the current density (FRG Pat. No. 1,006,547). These methods are suitable for welding workpieces with a small welding section, when the welding can be accomplished at a relatively high set specific power of welding machines.
None of the above methods has, however, found practical application for welding thick-walled workpieces with large welding sections. An adequate quality of joints is in this case attained by the welding method described in the works by Kuchuk-Yatsenko S. I. and Lebedev V. K. "Kontaktnaya svarka nepreryvnym oplavleniem izdeliy s bolshim poperechnym secheniem" (Continuous Resistance Flash Butt Welding of Large Cross-section Workpieces), Kiev, "UkrNIINTI", 1968, and "Kontaktnaya stykovaya svarka nepreryvnym oplavleniem" (Continuous Resistance Flash Butt Welding), Kiev, "Naukova dumka", 1976, p. 134. The method essentially consists in that the workpieces being welded are first fused at an initial constant speed of feeding the workpieces towards each other, and before the upsetting the feeding speed is stepwise or continuously increased from the initial value v.sub.3 to the final value v.sub.o.
If the fusion stability gets upset, the initial constant speed v.sub.3 can be appropriately adjusted in the course of welding.
During the period of fusion at the initial constant feeding speed which ordinarily is of 0.1 to 0.3 mm/s, the fusion is of a low intensity and proceeds with long current interruptions, with the result that a large amount of oxides originates on the surfaces being fused. The metal oxidation is particularly intense at the areas of deep craters, where the gap between the workpieces being welded is the maximum. It is at these areas that the thickest layers of oxides, most difficult to remove in the upsetting are formed.
As the feeding speed increases, so does the fusion intensity. Metal oxidation by the air oxygen in the welding zone (in the spark gap) diminishes. Therefore, increasing the feeding speed to the optimum value before the upsetting, in accordance with the method under consideration, allows the weld joint quality to be significantly upgraded.
The method enables an adequate quality of joints to be attained with either a stepwise (FIG. 1, a and b) or a continuous (FIG. 1, c and d) increase of the feeding speed. Both extensive theoretical studies and a vast body of practical evidence have shown that the conditions for producing high-quality joints are the most favourable when the increase of the feeding speed is governed by a wired-in program.
The above-described method, however, suffers from a disadvantage consisting in that the duration .tau..sub.1 of the period of the increased feeding speed v.sub.2 (FIG. 1) is set as a function of energy characteristics (e.g. current, power) of the fusion process, while no account is taken of the size of the maximum gap where oxide formation during the period of fusion at a constant initial speed is most probable. As a result, the desired effect, i.e. creation of the conditions for producing high-quality joints, is not in all cases attained for the period of an increased feeding speed.
This is especially pronounced in welding workpieces with a large, extended cross-section, where the fusion is to a considerable extent localized along the perimeter of the workpieces. This, especially with small upsetting tolerances, gives rise to defects such as thin oxide films at some regions of the joint zone, which in testing the weld joints shows up as a drop in the average mechanical characteristics, first of all in the plasticity characteristics.
At the same time, ever more exacting requirements upon the service characteristics of weldments, such as of large (up to 1420 mm) diameter gas pipelines at North, place particularly stringent demands on the mechanical properties of welded joints.